Lithos 109 (2009) 145–154 Contents lists available at ScienceDirect Lithos journal homepage: www.elsevier.com/locate/lithos The Sandvik peridotite, Gurskøy, Norway: Three billion years of mantle evolution in the Baltica lithosphere Thomas J. Lapen a,⁎, L. Gordon Medaris Jr.b, Brian L. Beard b, Clark M. Johnson b a Department of Geosciences, University of Houston, Houston TX 77204-5007, USA b Department of Geology and Geophysics, University of Wisconsin–Madison, Madison WI 53704, USA article info abstract Article history: The Sandvik ultramafic body, Island of Gurskøy, Western Gneiss Region, Norway, is a mantle fragment that Received 14 February 2008 contains polymetamorphic mineral assemblages and affords a unique view into the response of subcontinental Accepted 17 August 2008 lithospheric mantle to repeated orogenic/magmatic events. The Sandvik peridotite body and nearby outcrops Available online 6 September 2008 record four paragenetic stages: 1) pre-exsolution porphyroclasts of ol+grt+opx (high-Ca )+cpx (low-Ca), which equilibrated at 1100–1200 °C and 6.5–7.0 GPa; 2) kelyphite containing ol+grt+spl+opx (low-Ca)+am Keywords: (high-Al), as well as exsolved pyroxene containing opx+cpx+spl in equilibrium with matrix olivine, at 725 °C Lithospheric mantle Geothermobarometry and 1.5 GPa; 3) granoblastic matrix of ol+spl+opx (low-Ca)+am (high-Al), at 700 °C and 1.0 GPa. A nearby – – Sm–Nd geochronology outcrop contains a fourth assemblage consisting of ol+chl+opx+am. Lu Hf and Re Os model ages of garnet Western Gneiss Region peridotite indicate melt depletion at 3.3 Ga [Beyer, E.E., Brueckner, H.K., Griffin, W.L., O'Reilly, S.Y., Graham, S., Peridotite 2004. Archean mantle fragments in Proterozoic crust, Western Gneiss Region, Norway. Geology 32, 609–612.; Lapen, T.J., Medaris, L.G. Jr., Johnson, C.M., and Beard, B.L., 2005. Archean to Middle Proterozoic evolution of Baltica subcontinental lithosphere: evidence from combined Sm–Nd and Lu–Hf isotope analyses of the Sandvik ultramafic body, Norway. Contributions to Mineralogy and Petrology 150, 131–145.], marking the time of separation from the convecting mantle. Lu–Hf whole rock and mineral isochron ages of constituent garnet peridotite and garnet pyroxenite layers in the Sandvik body reflect cooling and emplacement at ~1.25 Ga and ~1.18 Ga, respectively, whereas Sm–Nd whole rock and mineral ages of the garnet pyroxenite layers and the garnet peridotite are consistent with metasomatic alteration at ~1.15 Ga [Lapen, T.J., Medaris, L.G. Jr., Johnson, C.M., and Beard, B.L., 2005. Archean to Middle Proterozoic evolution of Baltica subcontinental lithosphere: evidence from combined Sm–Nd and Lu–Hf isotope analyses of the Sandvik ultramafic body, Norway. Contributions to Mineralogy and Petrology 150, 131–145.]. The isochron ages likely record lithospheric modification associated with the 1.25–1.00 Ga Sveconorwegian orogeny and represent the youngest age of the Stage 1 mineral assemblage equilibration. A 606±39 Ma Sm–Nd isochron age of the Stage 2 kelyphite assemblage is consistent with partial re-equilibration of the porphyroclastic assemblage during continental rifting associated with opening of the Iapetus Ocean between Baltica and Laurentia at ~600 Ma, or extension between Baltica and Siberia that may have been associated with opening of the Ægir Sea. The age of kelyphite, therefore, places the Sandvik peridotite in the uppermost mantle prior to Silurian shortening between the Baltic and Laurentian continents. © 2008 Elsevier B.V. All rights reserved. 1. Introduction orogenic ultramafic bodies, which are commonly exposed in the high and ultra-high pressure terranes of collisional mountain belts, are The initial development of continents, their growth, and modifica- critical for understanding lithosphere evolution because they can tion are often recorded in the thermal, structural, and geochemical preserve mantle structures, pristine mineral assemblages, and/or evolution of subcontinental lithospheric mantle (SCLM). It is lithologic associations that may not be preserved in their associated frequently the case that mantle-derived xenoliths, peridotite massifs, crustal rocks or in mantle xenoliths alone (Medaris, 1999; Brueckner and orogenic mantle fragments show considerable variation in age, et al., 2002, 2004; Beyer et al., 2004, 2006; Lapen et al., 2005; Medaris chemical composition, and mineralogy, reflecting large-scale tecto- et al., 2006; Wittig et al., 2007). Furthermore, orogenic peridotites nothermal events that may also be recorded in superjacent crust record crust–mantle interactions that occur during continental (Medaris, 1999; O'Reilly et al., 2001; Lee, 2006). Many mantle-derived subduction and place critical constraints on the geodynamics of ultra-deep subduction (van Roermund et al., 2002), as well as the ⁎ Corresponding author. Tel.: +1 713 743 6368; fax: +1 713 748 7906. nature of rifting processes and continental break-up (Lemoine et al., E-mail address: [email protected] (T.J. Lapen). 1987; Manatschal, 2004; Montanini et al., 2006). In this paper, we 0024-4937/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.lithos.2008.08.007 146 T.J. Lapen et al. / Lithos 109 (2009) 145–154 present new age and petrologic data of the Sandvik peridotite body, et al., 1979; Tucker et al., 1990; Austreim et al., 2003). Following this Island of Gurskøy, Western Gneiss Region, Norway which records 3 period of continental growth by magmatic addition and terrane billion years of continental lithosphere growth and modification from accretion is a period of continental rifting, intrusion of dolerite dikes Middle Archean to Paleozoic time. and gabbros, and bimodal volcanism in the period between 1500 and 1250 Ma (Gorbatschev, 1980; Mörk and Mearns, 1986; Mearns, 1986; 1.1. Geological setting Gaal and Gorbatschev, 1987). The period from 1250–900 Ma repre- sents an interval of intense deformation, amphibolite to granulite- The Western Gneiss Region of Norway (WGR) represents the facies metamorphism, and magmatism associated with the Sveco- structurally lowest unit of the Scandinavian Caledonides and is norwegian orogeny (Austreim et al., 2003). Subsequent to, and along composed predominantly of amphibolite- to granulite-facies ortho- the strike of the Sveconorwegian orogeny, a major episode of rifting and paragneiss, pods and lenses of variably retrogressed peridotite, evolved between ~750 and 550 Ma, which generated part of the and eclogite (Fig. 1; Krogh and Carswell, 1995; Medaris, 1984; Iapetus Ocean between the margin of Baltica and Laurentia (Torsvik Carswell, 1981). A characteristic feature of the WGR is that measured et al., 1996; Dalziel, 1997; Bingen et al., 1998; Cawood and Pisarevsky, mineral ages of mantle-derived garnet peridotites (Proterozoic) are 2006; Cawood et al., 2007) or part of the Ægir Sea (Torsvik and much older than those of eclogites (Silurian) within the same Rehnström, 2001) between Siberia and an inverted Baltica (Hartz and tectonostratigraphic unit (see Brueckner and Medaris, 2000, for a Torsvik, 2002; Cocks and Torsvik, 2005). This rifting event produced discussion). The preservation of Proterozoic and older ages in mantle- extensive mafic dike swarms (Bingen et al., 1998) and resulted in derived garnet peridotites offers a rare opportunity to understand thinning of the continental margin. The thinned continental litho- ancient mantle events. Because of this, the mode and timing of sphere is likely the ‘protolith’ to components of the Seve-Køli Nappe eclogite and peridotite emplacement in gneiss, relative to high Complex (Hacker and Gans, 2005) which, like the WGR, contains pressure and ultrahigh pressure metamorphism associated with the Archean mantle fragments (Brueckner et al., 2004). The Caledonian Scandian phase of Caledonian orogeny (420–380 Ma), has been the orogeny, a product of closure of the Iapetus Ocean and continent– subject of considerable study (Lappin and Smith, 1978; Smith, 1980; continent collision between Baltica and Laurentia, resulted in Griffin et al., 1985; Cuthbert and Carswell, 1990; Wain, 1997; Wain eclogite–facies metamorphism across much of the WGR (See Cuthbert et al., 2000). Although recent evidence favors in situ eclogite-facies et al., 2000). This high- and locally ultrahigh-pressure (HP/UHP) metamorphism of metabasic lenses within enclosing quartzofelds- metamorphism is a product of continental subduction during the pathic gneisses (for reviews, see: Cuthbert and Carswell, 1990, and Scandian phase of the Caledonian orogeny in the interval of ~420– Krogh and Carswell, 1995), it is uncertain whether peridotite resided 380 Ma (Root et al., 2005; Kylander-Clark et al., 2000) as well as earlier in the crust or mantle prior to the Caledonian orogeny. HP/UHP events in the interval between 500 and 450 Ma (Brueckner The WGR is generally considered part of the Baltic Shield and has and van Roermund, 2004). At present, the petrologic and structural experienced several orogenic and magmatic events from about 1750 features in the WGR are the result of Caledonian reworking of earlier to 360 Ma (Gee and Sturt, 1985; Kullerud et al., 1986; Gaal and assembled lithologies. Gorbatschev, 1987; Andersen and Sundvoll, 1995), and this long history is likely imprinted upon the currently enclosed peridotite 1.2. Garnet peridotites bodies. These events include the 1750–1500 Gothian orogeny, which represents a period of extensive continental growth and magmatism. Peridotite bodies are distributed across the WGR (Fig. 1 MAP) and Gothian-age magmatic rocks represent some of the protoliths to the range in size from a few meters to several kilometers in exposed granitic gneisses in the WGR (Pidgeon and Raaheim, 1972; Lappin extent. There are two main types of peridotite based on major element Fig. 1. Location map of part of the Western Gneiss Region, modified after Carswell and Cuthbert (2003). T.J. Lapen et al. / Lithos 109 (2009) 145–154 147 Fig. 2. Summary of tectono-magmatic events recorded in the WGR (upper panel) and whole rock model and mineral isochron ages of Mg–Cr peridotites from the WGR (lower panel).